As a result of various industrial activities over the last 50 years or so by or on behalf of federal governments and also by private industry for its own purposes, numerous toxic waste sites have been identified, both in the United States and abroad. As the world's citizenry has become more knowledgeable about environmental matters, remediation of toxic waste sites has become a top priority for government. It is one objective of remediation to remove the toxins from on-site substrates, thereby rendering the substrates safely reusable. It is another objective to produce a reduced volume of concentrated and often still hazardous toxin for further treatment or final disposition. In favorable cases the process employed to remove the toxin also reduces or eliminates its toxicity. In general, the remediation of a toxic waste site presents significant technical challenges.
The nature of the toxic waste to be treated varies with the site. In some cases the toxin falls into a single category, such as heavy metals. Each category of toxin can require a process tailored to that category, and species-specific processes within a single category are necessary in some cases. Quite often the toxin consists of a mixture of chemical entities from different toxin categories. It can be especially difficult to separate such mixed toxins from the substrates upon which or in which the toxins are found.
In addition to the difficulties presented by the fact that different categories of toxins present different processing problems, there also are many different substrate types, so each toxic waste can present unique handling demands. For example, the decontamination of a soil, earth or dirt substrate contaminated with a toxin presents much different handling and processing requirements than the decontamination of workers' clothing contaminated with the same toxin.
A series of U.S. Patents held by the assignee of the instant application disclose and claim methods for removing toxins representative of many different categories from various substrates. Several of these patents are cited in the following paragraphs, their full texts being incorporated herein by reference. A common feature of these methods is that they employ similar chemistry, viz., "nitrogenous base/solvated electron" chemistry, the meaning of which is set forth in the prior art and in summary form hereinafter.
The term "nitrogenous base" is synonymous with the terms "ammoniacal liquid" and "a liquid ammonia" in the prior art and includes anhydrous liquid ammonia, amines, etc. Solvated electrons can be generated in the nitrogenous base electrolytically or by the addition of an active metal, such as sodium.
Several of the assignee's patents involve contaminated soil as the toxic waste. As a substrate or carrier type, "soil" is taken to have its ordinary meaning; soil includes one or more components in widely varying amounts, such as, clay, sand, stone, rock particles, organic matter, silt, water, etc.
Radioactive nuclides constitute one toxin category, and nuclear waste in which the toxin comprises at least one radioactive nuclide constitutes one type of toxic waste. The term "radioactive nuclide" includes the radioactive isotopes of elements comprising the Periodic Table of the Elements, radioactive compounds containing those elements, as well as both radioactive and non-radioactive products of their nuclear fission. Elements exhibiting radioactive isotopes include, for example, carbon, cesium, cobalt, potassium, plutonium, uranium, ruthenium, tellurium, thorium, strontium, rubidium, yttrium, rhenium, rhodium, palladium, technetium, neptunium, and americium. Nuclear waste comprising radioactive nuclides mixed with absorbed, or adsorbed in soil has been produced as a result of military testing programs involving the detonation of nuclear devices in the western deserts of the United States and elsewhere, leading to vast areas of real estate carrying hazardous nuclear waste.
U.S. Pat. No. 5,495,062 discloses and claims a process for separating radioactive nuclides from soil. The process involves slurrying the contaminated soil with ammonia, optionally containing solvated electrons. After the slurry settles, the more dense soil particles collect as a predominantly solid phase at the bottom of the slurry, while the less dense soil fines and nuclear waste are concentrated in the predominantly fluid upper phase. The decontaminated soil is readily recovered. Although solvated electrons need not be present, their presence improves layer demarcation in the slurry.
Heavy metals constitute another toxin category. "Heavy metals" are generally regarded as those metals which have a density of at least about 5.0 g/cm.sup.3. The "heavy metals" category includes, e.g., non-radioactive isotopes of mercury, arsenic, selenium, cadmium, chromium, and lead. Heavy metals are particularly difficult to eradicate from soil because they form water-soluble salts which are carried by sewage streams, rain water, etc., to locations distant from the original sites of their introduction into the environment.
Pursuant to the teachings of U.S. Pat. No. 5,516,968, elemental mercury can be separated from soil by slurrying the soil with an ammoniacal liquid and allowing the slurry to settle, whereupon the very dense mercury coalesces beneath a layer of the densest soil particles, and the soil fines suspended in the ammoniacal liquid form an upper phase. The decontaminated soil can be recovered after draining off the mercury bottoms product and decanting off the upper liquid layer.
Mercury ions, as well as other heavy metal ions, can be cleaned from contaminated soils by methods disclosed in U.S. Pat. No. 5,613,238. The contaminated soil is slurried with an ammoniacal liquid. Upon settling, the slurry is separated into a lower phase containing relatively dense soil particles and an upper liquid phase containing soil fines and the heavy metal ions. Decontaminated soil is readily recovered from the lower phase. It has been speculated that the ammoniacal liquid facilitates dissolution of the heavy metal ions in the upper liquid phase by forming amine coordination complexes with the metal ions.
The methods of the aforecited patents are extended to the decontamination of soils containing mixtures of toxins from different categories. For example, if the toxic waste consists of soil contaminated with both elemental mercury and one or more radioactive nuclides, slurrying the contaminated soil with an ammoniacal liquid, followed by phase separation, results in the liquid mercury being obtained as a bottom product, while the nuclear waste is found in the upper liquid layer with the soil fines; a middle layer comprises the decontaminated soil particles and is readily recovered.
Halogenated organic compounds represent another toxin category. "Halogenated organics" include compounds containing both carbon and halogen. The category encompasses a number of pesticides and herbicides, polychlorinated biphenyls (PCBs) employed as dielectrics in electrical transformers, dioxins, as well as cleaning fluids, such as trichloroethylene, etc. Pursuant to the teachings of U.S. Pat. Nos. 4,853,040 and 5,110,364, substrates contaminated with one or more halogenated organics can be decontaminated by treating such toxic wastes with an ammoniacal liquid containing solvated electrons.
Other categories of toxins include polynuclear aromatic hydrocarbons, volatile organic compounds (VOC's), and sludges. The "polynuclear aromatic hydrocarbons" category includes, for example, naphthalene, anthracene, phenanthrene, naphthacene, pyrene, perylene, etc., all of which are solids at room temperature, and some of which are believed to be carcinogenic. "VOC's" include the common organic solvents, such as benzene, toluene, hexane, heptane, acetone, methylethyl ketone, diethyl ether, methyl cellusolve, etc., which generally are flammable liquids having relatively high vapor pressures at room temperature. There are several types of "sludge," a generic term including, for example, sewage sludge as well as the intractable tarry bottoms products produced by various industrial processes, including the extraction of sugar from sugar beets, papermaking, oil refining, etc. The nitrogenous base/solvated electron technology can be applied in the remediation of toxic waste sites containing toxins from all of these categories.
Application of the nitrogenous base/solvated electron technology does not require elaborate facilities. Even though solvated electrons in nitrogenous base comprise one of the most powerful reducing agents ever produced, it is possible to use this reagent against toxic waste in the field even though the ubiquitous oxygen, water, etc. would be expected to overwhelm the toxins in competition for the available solvated electrons.
Although the cited prior art demonstrates that nitrogenous base/solvated electron technology is extremely effective in treating a broad range of toxins, it has been observed that solvated electrons can also react with solid carrier materials, such as soils and sludges holding the toxins, e.g., PCBs, resulting in inefficiencies in the reduction process. For example, the presence of organic matter, iron, clay, water, high cation exchange capacity, pH, carrier matrix itself, and so on, can render attempts to destroy toxins with solvated electrons economically unattractive. Competing side reactions occurring simultaneously with the desired reduction reaction with the targeted toxin can result in excessively high alkali metal requirements, making economical remediation of soils, for example, with solvated electrons impractical. Furthermore, the introduction of additional amounts of alkali metal or other active metal exceeding the stoichiometric requirements does not necessarily off-set the above inefficiencies, nor does it assure the reduction of toxins to minimum levels permitted under government regulations. Use of excess sodium can also result in poisoning the substrate/carrier matrix material.
Accordingly, there is a need for more efficient and cost effective protocols in the application of solvated electron technology in the decontamination of toxic waste and remediation of toxic waste sites.